1. Field of the Invention
The present invention relates to discrete multitone modulation (DMT), and more specifically, to the generation of cyclic prefixes in a DMT modulation transmission. The DMT modulation is for example used by standards ADSL and ADSL-lite.
2. Discussion of the Related Art
In a DMT modulation, data coded in the form of complex frequency coefficients are, on the transmit side, translated into time samples by inverse fast Fourier transform (IFFT).
FIG. 1 illustrates the IFFT of a group of N complex coefficients A1.ejφ1 to AN.ejφN. Each coefficient Ai.ejφi, where i is included between 1 and N, is associated with a respective frequency or tone fi. The transform of a coefficient Ai.eiφi is a sequence of digital samples in the time field, forming a section of a sinusoidal carrier of frequency fi, of amplitude Ai, and of phase φi. A first curve shows a sinusoid section of amplitude A1, of period 1/f1 and of phase φ1, obtained by IFFT of a coefficient A1.ejφ1 associated with a frequency f1. A second and a third curves show sections of sinusoids obtained by IFFT of coefficients A2.ejφ2 and AN.ejφN, respectively associated with frequencies f2 and fN.
An IFFT of the group of coefficients Ai.ejφi is formed by the sum of the sections of sinusoidal carriers obtained by IFFT of each of coefficients Ai.ejφi for i included between 1 and N, this sum being called a “symbol”. The IFFT of N coefficients provides a symbol Dt formed of a succession of N complex digital samples S1 to SN. It should be noted that the shape of the symbol Dt shown is not realistic, but aims at simplifying the under-standing of the present description.
The time samples obtained by IFFT are converted into analog to be transmitted, for example, by a telephone line. On the receive side, the analog signal of the line is converted into digital, and the resulting samples are converted into complex frequency coefficients by fast Fourier transform (FFT).
To suppress a number of problems due to interference between symbols appearing upon transmission of the symbols, a “cyclic prefix” (or guard interval) is interposed before each symbol. The cyclic prefix is the reproduction at the beginning of a symbol of the last samples of this symbol.
FIG. 2 shows a conventional circuit 10 of introduction of a cyclic prefix of τ samples. The complex coefficients Ai.ejφi for iε[1, N] are provided to an IFFT circuit 12. IFFT circuit 12 generates from the group of complex coefficients a symbol Dt comprised of N time samples S1 to SN. Symbol Dt is provided to a memory of FIFO type 14 and to a first input of a multiplexer 16. The output of memory 14 is connected to a second input of multiplexer 16.
At a time t1, IFFT circuit 12 provides a first sample S1 of symbol Dt, and memory 14 is controlled in the write mode to store this sample and the following. Multiplexer 16 is switched to select the output of memory 14, which provides a sample of a preceding symbol. This configuration of circuit 10 remains unchanged until a time tN−τ.
At time tN−τ, memory 14 has ended providing the samples of the preceding symbol and it contains the samples of the current symbol Dt, to the last sample preceding the cyclic prefix. IFFT circuit 12 starts providing the prefix samples, which samples, designated as SI to SN, continue being stored in memory 14. Meanwhile, multiplexer 16 is switched so that it transmits these prefix samples SI to SN. This configuration of circuit 10 remains unchanged until a time tN.
At time tN+1, IFFT circuit 12 is stopped, memory 14 contains the entire current symbol Dt and the prefix has just been transmitted. Multiplexer 16 is switched again to transmit the samples S1 to SN provided by memory 14, that is, symbol Dt.
At a time tN+τ+1, IFFT circuit 12 is reactivated and it starts providing the samples of the next sample. Time tN+τ+1 corresponds for the next symbol to previously-described time t1.
This configuration of circuit 10 remains unchanged until a time 2tN when symbol Dt will have been transmitted after its cyclic prefix.
Time 2tN+1 corresponds for the next symbol to previously-described time tN−τ.
A major disadvantage of circuit 10 is that the introduction of the cyclic prefix results in a delay tN (of N samples) in the transmission of symbol Dt. In some applications, such as telephone communications or other real time communications, the introduction of such a delay is not acceptable.
Besides, in prior art circuit 10, since the number N of samples may be high, memory 14 may have a large size.